1. Field of the Invention
The invention relates generally to an audio signal processing apparatus, and more particularly to an audio signal processing apparatus that does not interfere with reception of amplitude modulation broadcasts.
2. Description of the Related Art
Audio amplifiers, characterized by a wave output in the form of pulse width modulation (PWM), are commonly known as class D audio amplifiers. Class D audio amplifiers generate a high frequency square-wave output with harmonic distortion. When the output of a class D audio amplifier is connected to a load through an interconnecting line of a long length, then the interconnecting line operates as a transmitting antenna that could interfere with other electronic equipments. Specifically, amplitude modulation (AM) radios receive interference, which is undesirable.
Generally, the frequency band of the AM radio spans from 520 kilohertz (KHz) to 1.8 megahertz (MHz). However, the frequency band is on the base frequency of a switching frequency or AM radio harmonics from the class D audio amplifier. It has been found that serious interference with AM broadcasts is caused during operation of the class D audio amplifier. The radiated electric field of the class D audio amplifier has a greater magnitude than that of an AM broadcasting station from air when the class D audio amplifier is integrated with a tuner. Therefore, when the frequency band of the AM broadcasting station is near the switching frequency of the class D audio amplifier, the reception is intercepted by the electromagnetic radiation from the class D audio amplifier, thereby causing a so-called “jam” phenomenon. FIG. 1 is a diagram of a jam reception due to a clock signal Sc generated from a class D audio amplifier. As shown in FIG. 1, the second harmonic frequency 10 of the clock signal Sc from the class D audio amplifier falls on the receiving frequency F3. As a result, the receiving frequency F3 of the AM broadcasts is jammed and unable to be received. Additionally, side bands further extend, when the switching frequency of the class D audio amplifier shifts. Thus, as the number of the AM broadcast stations with interference increases, the problem where users cannot receive the AM broadcasts becomes more severe.
Consequently, there is a need for a class D audio amplifier for avoiding the conventional problems of interfering with AM broadcasts.